Various types of vibration sensors are presently known. These include, for example conductive balls which move under vibration to break a normally closed circuit and various spring loaded vibration sensitive devices. Vibration sensors employing mercury which at rest closes a circuit between two electrodes are also known. The known mercury vibration sensors define two states, at rest, when the circuit is closed and disturbed when the circuit is open.
Vibration sensors of the type described above and protective systems employing such sensors suffer from the significant disadvantage that individual sensors may be disabled, disconnected from the protective system or bypassed without providing a sensible indication to an operation.
Reflectometry is well known for use in locating transmission line faults. A general reference on reflectometry is "Time Domain Reflectometry" HP AN 75, published by Hewlett-Packard Inc., U.S.A. in March, 1966.